Amorphous silicon thin films are projected for use in diverse semiconductor applications. A high degree of photoconductivity makes this material ideally suited for photo-electric applications such as solar cells, photodetectors and the like. In thin film form, material costs for this semiconductor are sufficiently low to also allow its use in non-electronic applications such as solar thermal collectors or other applications utilizing the material characteristics of this semiconductor.
The efficiency of virtually all photo applications of amorphous silicon is dependent upon the incident photon entering into and being absorbed within the material. Absorption characteristics of bulk amorphous silicon are an inherent material property which cannot be significantly altered without altering other semiconductor properties. However, a percentage of incident photo flux may also be lost by reflection from the surface of the material. In general, reflection of incident light is a significant efficiency influencing factor in photo devices. For instance, when radiation is incident along a normal to an interface between two dielectric materials, one non-absorbing and the second absorbing, the ratio of the reflected to incident energy is calculable as: ##EQU1## where n is the refractive index of the first medium and n' is the refractive index of the second medium.
The bandgap of amorphous silicon varies relative to the amount of hydrogen within the film, resulting in a variance in the refractive index, n', from about 3.0 ev to about 3.5 ev (at .lambda.=2.0 microns). The refractive index is also dispersive with energy, evidencing a relatively constant value for .lambda. ranging from 2.0 microns to 1.0 microns, monotonically increasing to about 0.5 microns and decreasing thereafter. However, using an average value of n', an interface between air (having a value of n'.apprxeq.1) and amorphous silicon results in the reflection of approximately 35% to 45% of incident light energy.
Unlike absorption, there exists a variety of alternate techniques for reducing the reflectivity of surfaces. Semiconductor applications commonly employ an anti-reflection coating comprising a layer of generally transparent material having an optical index of refraction and layer thickness designed according to the expression: EQU n.sub.1 d.sub.1 =(2x-1).lambda.14 EQU n.sub.1 =(n.sub.o n.sub.2)
where n.sub.1 is the refractive index of the anti-reflection coating deposited onto a dielectric of index n.sub.2 ; x is an integer, .lambda. is the wavelength of the incident radiation and n.sub.o is the refractive index of the incident medium (typically air with a n.sub.0 .apprxeq.1.)
Although this technique is effective, it adds to the cost and complexity of the completed device. Furthermore, these anti-reflection coatings do not provide a "flat" or constant reduced reflectivity but are wave-length dependent. They are also critically dependent on controlled layer thickness.
An alternate technique is to texture or roughen the reflecting surface. This general technique may be divided into two categories, multiple internally reflective surfaces and gradient transition surfaces. The distinction between these two is physically one of size of the texturing, but phenomenalogically the two surfaces involve wholly distinct optical properties. The present invention relates generally to this latter category, this is, graded textured surfaces. Herein disclosed is a method for etching amorphous silicon in a hydrogen containing plasma to provide sub-micron dimensioning. In one embodiment, the geometry of the average etched cavity is of the same order of magnitude as the wavelength of incident light. An incident packet of light energy evidences a graded transition from a first optical medium (air for example) to the second medium, amorphous silicon. The graded transition reduces the reflectivity of the surface of amorphous silicon to below about 5%. Embodied in a photo-device such as a solar cell, the reduction in reflection losses from .apprxeq.40% to below about 5% correspondingly increases the efficiency of the device.
1. Field of the Invention
The present invention relates to amorphous silicon and more particularly to a method for plasma etching amorphous silicon surfaces to reduce its reflectivity of light energy.
2. Prior Art
The concept of reduced reflectivity textured surfaces is known in the art. Needle-like microstructures are typically either grown onto or etched into semiconductor surfaces by chemical treatments. Numerous techniques are known for forming these microstructures on semiconductor materials such as crystalline silicon and germanium.
Also known in the art are numerous techniques for producing amorphous silicon. Photoconductive amorphous silicon is generally produced by plasma decomposition of silane or, alternatively, by sputtering in a plasma containing argon and hydrogen. As noted heretofore, the conventionally deposited amorphous silicon film has a surface reflectivity of about 40%. The present invention teaches a method for micro-texturing this reflective surface by etching the amorphous silicon surface in a hydrogen containing plasma. In U.S. Pat. No. 4,151,058, Kaplan et al have treated amorphous silicon films in a hydrogen plasma to incorporate hydrogen into the silicon film. The method taught therein is directed to producing photoconductivity in the previously un-hydrogenated films. In contrast, the present invention does not diffuse hydrogen into the amorphous silicon film but operates to etch the silicon.